The present invention relates to a circuit and method for driving a liquid-crystal display, such as an active-matrix display employing thin-film transistors.
Active-matrix liquid-crystal displays or LCDs are widely employed in, for example, portable computers, where they provide the advantages of high-speed response and reduced crosstalk. In a typical active-matrix LCD, for each of the three primary colors, each picture element or pixel has a thin-film transistor (TFT) that is switched on and off by a signal received from a gate line, and a liquid-crystal capacitor that charges or discharges through a source line when the TFT is switched on. The source and gate lines form a matrix in which the gate lines are activated one at a time, and the source lines carry signals representing the displayed intensities of the picture elements. In the alternate-current or AC driving system that is usually employed with active-matrix LCDs, adjacent liquid-crystal capacitors are charged in opposite directions centered around a common potential. A more detailed description of the active-matrix circuit configuration and AC driving scheme will be given later.
In the AC driving scheme, as successive gate lines are activated, the voltage of each source line must swing alternately above and below the common potential. The voltage swings on the source lines are thereby doubled, as compared with direct-current driving. A resulting problem is that the time needed to charge the parasitic capacitances of the source lines is increased, current consumption is similarly increased, and large source-line driving circuits are needed. The large charging and discharging currents furthermore generate electrical noise.
Although the gate lines are not driven in an AC manner, they also have parasitic capacitances that must be charged and discharged. The charging and discharging of the gate lines similarly takes time, consumes current, generates noise, and requires large driving circuits.